The present invention relates to a method and system for a signal transmission for transmitting a component video signal, a sound signal, and a data signal through, in particular, a single core optical fiber.
In order to transmit a component video signal through a single core optical fiber, one of the following methods, that is, a frequency multiplex method in which respective signals are frequency multiplexed after the signals have been modulated by different frequencies, or a PCM multiplex method in which respective signals are multiplexed after the signals have been PCM encoded, or a wavelength multiplex method in which respective signals are transmitted to a light emitting device by using three different wavelengths has been employed.
FIGS. 1A to 1D show block diagrams of transmitter sections of these methods.
FIG. 1A shows the frequency multiplex method. A luminance signal (hereinafter referred to Y, or a Y signal) and two color difference signals (referred to as Pr or a Pr signal, and Pb or a Pb signal respectively) are respectively modulated by three different frequencies f0, f1, and f2 in modulators 100, 101, and 102, and then the modulated signals are frequency multiplexed by a multiplexer 103. The frequency multiplexed signal is converted to an optical signal by an electro-optic converter (hereinafter, referred to as an E.O) 104, and delivered to an optical fiber. As a typical one of such a multiplexer, there is a component/composite signal converter. When a wide band, high definition color television signal (hereinafter, referred to as a HDTV signal) is to be transmitted by this method, a light emitting device having a wide band and satisfactory linearity is required as the E/O 104, and the selection of the light emitting device is difficult.
In FIG. 1B, a pulse modulater 114 is connected between the output of the multiplexer 103 and the E/O 115. As the pulse modulater 114, there are a pulse frequency modulater, a pulse width modulater, and the like. In this method, although the linearity of an E/O 115 is not required, the pulse modulater 114 having a wide band and satisfactory linearity is required.
FIG. 1C shows the PCM multiplex method. The Y, Pr, and Pb signals are respectively converted to digital signals by PCM encoders 105, 106, and 107. These three digital signals are time division multiplexed by a multiplex section 108. The time division multiplexed signal is converted to an optical signal by an E/O 109, and delivered to an optical fiber. The transmission speed of the E/O 109 is equal to several hundreds of M bps or larger. In this method, the scale of the system is large and the cost is high, and furthermore, the optical fiber for a transmission line is required to be of very wide band.
FIG. 1D shows the wavelength multiplex method. The Y, Pr, and Pb signals are respectively inputted to E/O's 110, 111, and 112 including light emitting devices which have wavelengths of respectively .lambda.1, .lambda.2, and .lambda.3, and the inputted signals are converted to optical signals. These optical signals are multiplexed by an optical combiner 113, and delivered to an optical fiber. This method requires many kinds of light emitting devices, and optical equipment including optical combining and branching devices, etc. Furthermore, the velocities of propagation of the optical signals having wavelengths of .lambda.1, .lambda.2, and .lambda.3 in the optical fiber differ from one another because of a difference in refractive index due to the wavelengths of the light emitting devices. Accordingly, a drawback is involved in that a relative phase of the signals is different depending on the transmission distance, and the correction thereof is difficult.
As described above, the related arts are methods involve the above-mentioned drawbacks respectively in transmitting the component signal having a wide band, in particular, such as a HDTV signal.